Urdu poetry is full of recurring themes, covering a vast number of topics. Collecting sher-o-shaayari has been a hobby since childhood. Categorizing them and arranging them by theme was a logical extension. This was the genesis of Unwaan, a thematic collection of Urdu couplets or ashaar (ashaar = plural of sher).
The list is not exhaustive by any means. If you don't see your favourite sher for any of the listed topics or a topic itself, please feel free to contribute. Suggestions, additions, corrections are welcome as well.